I mean, "don't trust the client" was a thing in the late '90s when people were exploiting client-controlled things in Ultima Online so much Origin had to change the way some communication worked to stop it.
The most famous example I can think of was UOExtreme revealing hidden players, because if a player was hiding their presence was still sent to the clients of every other player in the area just with a "hidden" flag set. There were a bunch of other similar exploits associated with that particular third-party tool, but that's the only one I remember.
(so while writing this I did some googling and found the patch notes where they fixed it: https://uo.com/wiki/ultima-online-wiki/technical/previous-pu...)
